In general, an organic light emitting device includes two electrodes and an organic compound layer interposed between the electrodes. In the organic light emitting device, electrons and holes are injected into the organic compound layer from the two electrodes, and a current is converted into visible light. In the organic light emitting device, in order to improve performance, an electron/hole injection layer or an electron/hole transport layer may be further provided, in addition to the organic compound layer for converting the current into visible light.
However, an interface between the electrode formed of metal, metal oxides, or conductive polymers and the organic compound layer is unstable. Accordingly, heat applied from the outside, internally generated heat, or an electric field applied to the device has an adverse effect on performance of the device. Further, a driving voltage for device operation may be increased due to a difference in conductive energy level between the electron/hole injection layer or the electron/hole transport layer and another organic compound layer adjacent thereto. Accordingly, it is important to stabilize an interface between the electron/hole injection layer or the electron/hole transport layer and another organic compound layer. It is also important to make injection of electrons/holes easy by minimizing an energy barrier for injection of electrons/holes from the electrode to the organic compound layers.
The organic light emitting device has been developed so as to adjust a difference of energy level between two or more electrodes and an organic compound layer interposed between the electrodes. For example, an anode is adjusted to have a Fermi energy level similar to an HOMO (highest occupied molecular orbital) energy level of a hole injection layer or a material having an HOMO energy level similar to a Fermi energy level of an anode is selected for a hole injection layer. However, since the hole injection layer needs to be selected in view of an HOMO energy level of a hole transport layer or a light emitting layer close to the hole injection layer as well as in view of the Fermi energy level of the anode, there is a limitation to select a material for the hole injection layer.
Accordingly, in the method for manufacturing an organic light emitting device, a method of adjusting a Fermi energy level of an electrode is adopted. Materials for an anode are limited to materials having a high Fermi energy level of 5.0˜5.5 eV, for example ITO, IZO, Au, Ni, Mo and the like, because materials having an HOMO level of about 5.0˜5.5 eV are generally used as an organic compound layer that are adjacent to the anode. Materials for a cathode are also selected to have a proper Fermi energy level depending on an LUMO energy level of an electron transport layer that transports electrons. An electron transport layer generally has LUMO level of about 3.0 eV, and thus materials having a Fermi energy level less than 3.0 eV are preferably used for cathode materials. Examples of materials for a cathode include lithium (Li), calcium (Ca), magnesium (Mg) and the like. However, most of the above materials are unstable. Therefore, metals having a relatively high Fermi energy level such as aluminum (Al), silver (Ag) and the like are used for a cathode, while an electron injection layer between an electrode and an electron transport layer is used in order to improve injection of electrons. Materials such as LiF, NaF, KF and the like can be used for the electron injection layer and the above materials are known to reduce the energy barrier for electron injection to an electron transport layer.
Electrode materials are limited to being used as anode materials or cathode materials depending on their Fermi energy level in order to improve charge injection into organic compounds. While transparent electrode materials having a high Fermi energy level such as ITO, IZO and the like can be used as anode materials, materials having a low Fermi energy level and a high reflectivity such as Al, Ag and the like can be used as cathode materials. Due to the limitation in the selection of electron materials, most of organic light emitting devices have a structure in which light is emitted through a transparent anode. Recently, it is required to emit light through a cathode, and thus anode materials having a high reflectivity and cathode materials having a high transparency are required. Also, in the development of transparent OLEDs that emit light both sides through an anode and a cathode, the necessity of cathode materials having an excellent transparency has increased. Materials that have a low Fermi energy level and a semi-transparent property when they are deposited into a thin film such as Mg, Ag, MgAg, Ca, CaAg and the like are used as transparent cathode materials. The semi-transparent cathode materials have a problem in that their conductivity decreases because electrodes have to be formed with thin films in order to improve their transparency.